[PATCH v2] vc_screen: modify vcs_size() handling in vcs_read()

[PATCH v2] vc_screen: modify vcs_size() handling in vcs_read()

[George Kennedy]

Restore the vcs_size() handling in vcs_read() to what
it had been in previous version.

Fixes: 226fae124b2d ("vc_screen: move load of struct vc_data pointer in vcs_read() to avoid UAF")
Suggested-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: George Kennedy <george.kennedy@oracle.com>
---
v2: added Fixes

 drivers/tty/vt/vc_screen.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
index f566eb1839dc..c0a2273bb998 100644
--- a/drivers/tty/vt/vc_screen.c
+++ b/drivers/tty/vt/vc_screen.c
@@ -414,10 +414,8 @@ vcs_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 		 */
 		size = vcs_size(vc, attr, uni_mode);
 		if (size < 0) {
-			if (read)
-				break;
 			ret = size;
-			goto unlock_out;
+			break;
 		}
 		if (pos >= size)
 			break;
-- 
2.31.1

Re: [PATCH v2] vc_screen: modify vcs_size() handling in vcs_read()

[Linus Torvalds]

On Mon, Feb 27, 2023 at 12:22 PM George Kennedy
<george.kennedy@oracle.com> wrote:
>
> Restore the vcs_size() handling in vcs_read() to what
> it had been in previous version.

I took this one directly since I'd been involved in the discussion the
whole time, so we can just close this issue.

           Linus

Re: [PATCH v2] vc_screen: modify vcs_size() handling in vcs_read()

[linux]

Hi,

On Mon, Feb 27, 2023 at 03:21:41PM -0500, George Kennedy wrote:
> Restore the vcs_size() handling in vcs_read() to what
> it had been in previous version.

This still seems to be broken for me.

Testcase:

# cat /dev/vcsa1
[.. data, looks complete ..]
cat: /dev/vcsa1: No such device or address

"ret" is still unconditionally overwritten with -ENXIO at the beginning
of the loop.
And when we break the loop because everything has been read in
`if (pos >= size)` then this error is returned to userspace instead of
just `0`.

I still need the patch from 

https://lore.kernel.org/lkml/20230220064612.1783-1-linux@weissschuh.net/

> Fixes: 226fae124b2d ("vc_screen: move load of struct vc_data pointer in vcs_read() to avoid UAF")
> Suggested-by: Jiri Slaby <jirislaby@kernel.org>
> Signed-off-by: George Kennedy <george.kennedy@oracle.com>
> ---
> v2: added Fixes
> 
>  drivers/tty/vt/vc_screen.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
> index f566eb1839dc..c0a2273bb998 100644
> --- a/drivers/tty/vt/vc_screen.c
> +++ b/drivers/tty/vt/vc_screen.c
> @@ -414,10 +414,8 @@ vcs_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>  		 */
>  		size = vcs_size(vc, attr, uni_mode);
>  		if (size < 0) {
> -			if (read)
> -				break;
>  			ret = size;
> -			goto unlock_out;
> +			break;
>  		}
>  		if (pos >= size)
>  			break;
> -- 
> 2.31.1
> 

Re: [PATCH v2] vc_screen: modify vcs_size() handling in vcs_read()

[Linus Torvalds]

On Mon, Feb 27, 2023 at 5:46 PM <linux@weissschuh.net> wrote:
>
> This still seems to be broken for me.

Looks that way.

> I still need the patch from
>
> https://lore.kernel.org/lkml/20230220064612.1783-1-linux@weissschuh.net/

.. and that has the same problem with "what if the error happens
during an iteration that wasn't the first, and we already succeeded
partially".

The "goto unlock_out" is bogus, since it jumps over all the "update
pos and check if we read something".

It was the correct thing to do *above* the loop, but not inside the loop.

IOW, I think the proper patch is to also turn the "goto unlock_out"
into a "break". Mind testing something like this (whitespace-damaged,
but you get the idea):

    --- a/drivers/tty/vt/vc_screen.c
    +++ b/drivers/tty/vt/vc_screen.c
    @@ -403,10 +403,11 @@ vcs_read(struct file *file, char __user
*buf, size_t count, loff_t *ppos)
                unsigned int this_round, skip = 0;
                int size;

    -           ret = -ENXIO;
                vc = vcs_vc(inode, &viewed);
    -           if (!vc)
    -                   goto unlock_out;
    +           if (!vc) {
    +                   ret = -ENXIO;
    +                   break;
    +           }

                /* Check whether we are above size each round,
                 * as copy_to_user at the end of this loop

which hopefully really fixes this (at least I don't see any other
"goto unlock_out" cases).

              Linus

Re: [PATCH v2] vc_screen: modify vcs_size() handling in vcs_read()

[Thomas Weißschuh]

On Mon, Feb 27, 2023 at 05:58:12PM -0800, Linus Torvalds wrote:
> On Mon, Feb 27, 2023 at 5:46 PM <linux@weissschuh.net> wrote:
> >
> > This still seems to be broken for me.
> 
> Looks that way.
> 
> > I still need the patch from
> >
> > https://lore.kernel.org/lkml/20230220064612.1783-1-linux@weissschuh.net/
> 
> .. and that has the same problem with "what if the error happens
> during an iteration that wasn't the first, and we already succeeded
> partially".

Indeed.

> The "goto unlock_out" is bogus, since it jumps over all the "update
> pos and check if we read something".
> 
> It was the correct thing to do *above* the loop, but not inside the loop.
> 
> IOW, I think the proper patch is to also turn the "goto unlock_out"
> into a "break". Mind testing something like this (whitespace-damaged,
> but you get the idea):

Makes sense and seems to work correctly.

Tested-By: Thomas Weißschuh <linux@weissschuh.net>

(Or feel free to use my patch from above and fixup the goto/break line)

> 
>     --- a/drivers/tty/vt/vc_screen.c
>     +++ b/drivers/tty/vt/vc_screen.c
>     @@ -403,10 +403,11 @@ vcs_read(struct file *file, char __user
> *buf, size_t count, loff_t *ppos)
>                 unsigned int this_round, skip = 0;
>                 int size;
> 
>     -           ret = -ENXIO;
>                 vc = vcs_vc(inode, &viewed);
>     -           if (!vc)
>     -                   goto unlock_out;
>     +           if (!vc) {
>     +                   ret = -ENXIO;
>     +                   break;
>     +           }
> 
>                 /* Check whether we are above size each round,
>                  * as copy_to_user at the end of this loop
> 
> which hopefully really fixes this (at least I don't see any other
> "goto unlock_out" cases).
> 
>               Linus

Re: [PATCH v2] vc_screen: modify vcs_size() handling in vcs_read()

[Linus Torvalds]

On Mon, Feb 27, 2023 at 6:18 PM Thomas Weißschuh <linux@weissschuh.net> wrote:
>
> Tested-By: Thomas Weißschuh <linux@weissschuh.net>
>
> (Or feel free to use my patch from above and fixup the goto/break line)

Done.

              Linus